Grain size distribution has been treated extensively in the art because of its effect on photographic speed as well as grain surface area which relates to the absorption of sensitizing dye on the grain and the attendant effects of these factors in the various photographic products. In addition, it is well known that granularity is significantly affected by grain size. While many photographic products can satisfactorily employ silver halide emulsions possessing relatively wide grain size distributions, that is, appreciable numbers of grains of varying sizes, many applications find narrow grain size distribution silver halide emulsions preferable.
In the formation of photosensitive silver halide emulsions, the ripening or growing step during which time the silver halide grains grow is considered important. During the ripening stage the presence of an adequate concentration of a silver halide solvent, for example, excess halide, generally bromide, is employed which renders the silver halide much more soluble that it is in pure water because of the formation of complex ions. This facilitates the growth of the silver halide grains. While excess bromide and ammonia are the most common ripening agents, the literature also mentions the use of water-soluble thiocyanate compounds in place of bromide as well as a variety of amines. See, for example, Photographic Emulsion Chemistry, G. F. Duffin, The Focal Press London, 1966, page 59.
Zelikman and Levi, Making and Coating Photographic Emulsions, The Ford Press, N.Y. 1964, page 96, have stated that as time increases in first ripening or growth step of a neutral silver halide emulsion prepared with a large excess of bromide ion, the width of the grain size distribution curve increases as well as the average grain size. Thus, the distribution becomes progressively wider and is shifted into the coarse-grained region as ripening proceeds.
To avoid a widening of grain size distribution by Ostwald ripening, it is known in the art to employ a pAg feedback control system that prevents a significant excess of halide from being present during silver halide grain formation.
The art has also disclosed the employment of a water-soluble thiocyanate compound as being present during the formation of the grains, that is, during the actual precipitation of the photosensitive silver halide. For example, U.S. Pat. No. 3,320,069 discloses a water-soluble thiocyanate compound which is present as a silver halide grain ripener either during actual precipitation of the light sensitive silver halide or added immediately after precipitation. The precipitation of the siler halide grains in the aforementioned patent is carried out, however, with an excess of halide.
U.S. Pat. No. 4,046,576 is directed to a method for the continuous formation of photosensitive silver halide emulsions wherein a silver salt is reacted with a halide salt in the presence of gelatin to form a photosensitive silver halide emulsion and said formation takes place in the presence of a sulfur-containing silver halide grain ripening agent, such as a water-soluble thiocyanate compound, and the thus-formed silver halide emulsion is continuously withdrawn from the reaction chamber while silver halide grain formation is occurring. During precipitation the halide concentration in the reaction medium is maintained at less than 0.010 molar. The patent states that it is known in the art to prepare silver halide grains in the presence of an excess of silver ions. The patent relates to such a precipitation with the additional steps of continually adding the sulfur-containing ripening agent and continually withdrawing silver halide grains as they are formed.
U.S. Pat. No. 4,150,994 is directed to a method of forming silver iodobromide or iodochloride emulsions which are of the twinned type which comprises the following steps:
(a) forming a monosized silver iodide dispersion;
(b) mixing in the silver iodide dispersion aqueous solutions of silver nitrate and alkali or ammonium bromides or chlorides in order to form twinned crystals;
(c) performing Ostwald ripening in the presence of a silver solvent, such as ammonium thiocyanate, to increase the size of the twinned crystals and dissolve any untwinned crystals;
(d) causing the twinned crystals to increase in size by adding further aqueous silver salt solution and alkali metal or ammonium halide; and
(e) optionally removing the water-soluble salts formed and chemically sensitizing the emulsion.
A novel method has now been found for forming photosensitive silver halide emulsions with a narrow grain size distribution.